1. Field of the Invention
The invention relates to silicone polymers having ionic and/or organometallic functional groups, which can be crosslinked via coulombic interactions and possibly also via covalent bonds to form high-strength elastomers.
2. Description of the Related Art
The extraordinary importance of silicones as elastomeric materials is based both on their high thermal stability and their extreme low-temperature flexibility. Furthermore, they have excellent UV light stability and oxidation resistance. Silicones also have a unique combination of many other properties, for example a low dielectric constant, good leakage current resistance and dielectric breakdown resistance, hydrophobicity, optical transparency and biocompatibility. The Shore hardness of silicone elastomers can be set over a wide range.
Silicone elastomers are therefore used in numerous industrial sectors in a variety of applications. In automobile construction, they are used, for example, as vibration and resonance dampers, exhaust and catalyst suspensions and also, for example, as engine bearers. Further applications are peristaltic pump tubing in the medical sector, membranes, valves, seals and also, for example, insulators and cable end seals in the high-voltage sector.
However, in terms of their mechanical properties such as the tensile strength, tear propagation resistance and abrasion resistance, silicone elastomers are inferior to other organic elastomers. For this reason, only silicone elastomers which have been reinforced with fillers, in particular finely divided silica, come into consideration for industrial applications. Even then, the abovementioned strengths are comparatively low.
One possible way of improving the mechanical strength of a polymeric material is generally to introduce ionic groups, as is described, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, 4th ed., Vol. 14, pp. 815-829 (1995).
Silicone elastomers formed by crosslinking of ionic and/or organometallic polymers have hitherto not become widely known. In such silicone elastomers, it is possible for not only ionic and/or organometallic bonds but also covalent bonds to be present, as is the case, for example, for classical ionomers. Furthermore, silicone polymers which contain ionic or organometallic functions but are not crosslinked to form high-strength elastomers are known.
In U.S. Pat. No. 6,783,709, self-healing organosiloxanes which contain reversible and energy-dissipative crosslinking domains are described. In this case, linear polydimethylsiloxanes having lateral oligoglycine groups are prepared. The gel-like masses obtained have self-healing properties. For the present purposes, self-healing is a growing together of cut surfaces which have been brought into contact again after cutting of the gel. The crosslinking points which are responsible for this and are formed by noncovalent interaction have a bond strength of at least 50 and not more than about 1000 pN, which characterizes these as nonspecific interactions (London, Keesom and Debye forces) and hydrogen bonds. The further ionomers disclosed as crosslinker components are not specified in chemical terms. It is therefore not possible to see whether they are organic or silicone isonomers. The structure of these ionomers is said to be noncritical. Rather, these can vary in terms of their chemical composition, charge density and size as long as the silicone compositions claimed each contain ionomers having opposite charges. Compositions which may contain covalent crosslinking points are described, but not any covalently crosslinkable silicone compositions. The mechanical properties of these ionomers or products obtained therefrom are not described further.
The patent documents CA 2,209,486 and CA 2,274,040 describe silicones which have a hydrophobic polysiloxane backbone and at least one hydrophilic group which is covalently bound to the hydrophobic polysiloxane backbone. Hydrophilic groups are chelating ligands having at least two carboxyl functions. Representatives of this class of compounds are based on substituted malonic acid and N-substituted iminodiacetic acid and also, for example, many structural variants of the latter compound or EDTA analogues in general. These silicones having chelating functionality are used as metal-binding or metal ion-binding or surface-active substances. The silicones described are not covalently crosslinkable silicone compositions.
The preparation and properties of carboxylate-functional siloxane ionomers is described in G. A. Gornowitz et al., POLYM. MATER. SCI. ENG. 59, 1009-1013 (1988). These are polysiloxanes having carboxyl functions in the side chain and the corresponding salts of ions of the metals lithium, zinc, titanium, lead and calcium. It can be seen here that although the polysiloxane-containing ionomers have higher ultimate tensile strengths than the corresponding functional polymers containing pure carboxylate groups, i.e. without metal counterions, they at the same time have very low elongations at break, i.e. are very brittle materials which have only very limited usefulness as elastomers.
Curable elastomer compositions are described for the first time in the patent U.S. Pat. No. 3,047,528. These compositions comprise polysiloxane containing carboxyalkyl groups, a filler, for example silica, and polyvalent metal compounds as curing agents. Crosslinking to form an elastomer occurs exclusively via crosslinking points resulting from coulombic interactions between the metal and carboxylate ions. However, the mechanical strengths achieved are far below the values of silicone elastomers customary today. In addition, U.S. Pat. No. 3,248,409 discloses polyvalent metal salts of carboxyalkyl-functional polysiloxanes which are used for making textiles water-repellant.
Polydimethylsiloxanes having lateral carboxyl groups, and also the corresponding zinc ionomers are described by Klok et al., J. POLYM. SCI., PART B, POLYM. PHYSICS 37, 485-495 (1999). Here, the reversible gelling of polydimethylsiloxanes having ionic substituents and substituents which form hydrogen bonds is described.
Examples of liquid-crystalline polysiloxanes and ionomers having sulfonic acid groups in the side chain are disclosed in B. Zhang et al., J. APPL. POLYM. SCI. 68, 1555-1561, and in J. Hu et al., J. APPL. POLYM. SCI. 80, 2335-2340 (2001). Here, the influence of ion aggregation on the mesomorphic properties and the thermal properties of the liquid-crystalline polymers obtained are examined. The mechanical properties are not described.
European published specification EP 1 264 865 A1 claims silicone compositions having improved adhesion and cured silicone products produced therefrom. The compositions are addition-crosslinkable silicone rubbers which contain vulcanizable titanium or zirconium complexes of beta-ketocarbonyl compounds as adhesion promoters. The latter can optionally contain siloxane radicals having an average degree of polymerization of from 0 to 20.
Among the organometallic compounds, metallocenes, in particular ferrocene, are of particular importance. German published specification DE 14 95 970 A1 describes a polysiloxane containing metallocene groups. An HTV vulcanizate which contains such a ferrocene-siloxane copolymer as heat stabilizer is also described.
Z. Huang et al., J. APPL. POLYM. SCI. 83, 3099-3104 (2002) describe the ionic aggregation of polysiloxane ionomers having laterally bound, quaternary ammonium groups. These are compounds which are liquid at room temperature and are not crosslinked to form elastomers.
Proceeding from amino-functional siloxanes, it is possible to obtain zwitterionomers by reaction with propane sultone, as is described, for example, in D. Gravier et al., J. POLYM. SCI., POLYM. CHEM. ED., 17, 3559-3572 (1979). Depending on the concentration of the ionic groups, slight rubber-elastic properties are observed or flexible materials are obtained. The mechanical strengths are generally low and do not correspond to the state of the art of silicone elastomers customary today.
X. Yu et al., J. POLYM. SCI., PART B, POLYM. PHYSICS 24, 2681-2702 (1986) describe block copolymers of zwitterionomeric polysiloxanes and polyurethanes. The mechanical properties of the elastomers obtained are better the higher the proportion of polyurethane segments and the greater the ionic functionality present in the polysiloxane segment. However, pure polysiloxane zwitterionomers not also containing urethane copolymers display only low mechanical strengths.